Reagent for determining the absolute configuration of chiral compound and determination method

ABSTRACT

The present invention relates to a reagent for determining the absolute configuration of a chiral compound containing as an active ingredient a metalloporphyrin dimer, wherein the metalloporphyrin dimer has an alkaline-earth metal ion as a central metal and has a carbon chain-crosslinked structure in which at least one of the two porphyrin rings has a substituent bulkier than methyl at least one of the second carbon atoms from a carbon atom bonded to the crosslinking carbon chain along the outer periphery of the porphyrin ring and a method for determining the absolute configuration of an asymmetric carbon atom of the chiral compound using the reagent.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP03/00147, filed Jan. 10, 2003, whichclaims priority of Japanese Patent Application No. 2002-51505, filed onFeb. 27, 2002. The International Application was published under PCTArticle 21(2) in a language other than English.

TECHNICAL FIELD

The present invention relates to a reagent for determining the absoluteconfiguration of chiral compounds and a method for determining theabsolute configuration of chiral compounds using the reagent.

BACKGROUND OF THE INVENTION

Conventionally, the absolute configuration of chiral compounds has beendetermined by circular dichroism (CD) spectrophotometric measurement ofa complex of a chiral compound and a specific compound based on thecorrelation between the sign of the Cotton effect and the absoluteconfiguration of chiral compounds. For example, the following methodsare reported.

(1) X. Huang, B. H. Rickmann, B. Borhan, N. Berova, and K. Nakanishi (J.Am. Chem. Soc., 1998, 120, 6185-6186) report that circular dichroism isinduced when chiral compounds are coordinated to a longchain-crosslinked porphyrin dimer, and there is a correlation betweenthe sign of the Cotton effect and the absolute configuration of thechiral compounds.

In this system, however, circular dichroism is induced only when onechiral compound is simultaneously coordinated to two porphyrin units.Therefore, this method is useful only for bifunctional chiral compoundssuch as diamines, aminoalcohols, etc.

(2) M. Takeuchi, T. Imada, and S. Shinkai (Bull. Chem. Soc. Jpn., 1998,71, 1117-1123) report that a porphyrin dimer having a phenylboronic acidunit exhibits circular dichroism in the presence of various sugars.

This system is applicable only to sugars which bind to boronic acid, andit is not a system for directly determining the absolute configurationof any specific-asymmetric center among the many asymmetric centers ofsugars.

As is clear from the above, there have been no reports of a method fordetermining the absolute configuration applicable to a wide variety ofchiral compounds, such as monoalcohols, etc.

X-ray diffraction is a known method for determining the absoluteconfiguration of chiral compounds. However, the compounds to which thismethod is applicable are limited to crystalline compounds.

The present inventors have investigated methods for precisely and easilydetermining the absolute configuration of various chiral compounds. Inrecent years, they have found that circular dichroism is induced when achiral compound is coordinated to a metalloporphyrin dimer containingZn, Fe, Mn, or Ru as the central metals, and the sign of the Cottoneffect and the absolute configuration of the chiral compound arecorrelated with each other. A novel method for determining the absoluteconfiguration of chiral compounds was thus completed based on thisfinding (Japanese Unexamined Patent Publication No. 2001-220392).

However, even this method arises a problem such that the induced Cottoneffect cannot be detected in the case of a chiral compound of amonoalcohol, etc. unless the sample solution is cooled to about −80° C.

DISCLOSURE OF THE INVENTION

The present invention is made in view of the problems of the prior art,and the principal object is to provide a reagent which allows the easy(i.e., without the complicated operation of cooling, etc.),highly-sensitive and precise determination of the absolute configurationof chiral compounds such as monoalcohols, etc., and a method fordetermining the absolute configuration of chiral compounds using thereagent.

The present inventors conducted intensive research and found that theobjects can be attained by a reagent containing a specificmetalloporphyrin dimer as an active ingredient, and a method using thereagent, to thereby accomplish the present invention.

More specifically, the present invention relates to a reagent fordetermining the absolute configuration of chiral compounds and adetermination method thereof as described below.

-   1. A reagent for determining the absolute configuration of a chiral    compound containing as an active ingredient a metalloporphyrin    dimer,

wherein the metalloporphyrin dimer has an alkaline-earth metal ion as acentral metal and has a carbon chain-crosslinked structure in which atleast one of the two porphyrin rings has a substituent bulkier thanmethyl at at least one of the second carbon atoms from a carbon atombonded to the crosslinking carbon chain along the outer periphery of theporphyrin ring.

-   2. A reagent for determining the absolute configuration of a chiral    compound according to Item 1, wherein the carbon chain-crosslinked    metalloporphyrin dimer is a metalloporphyrin represented by formula    (1):

wherein M²⁺ and M′²⁺ are the same or different and each represent analkaline-earth metal ion,

n is 2 or 3,

R^(a) to R^(d) are the same or different and each represent a hydrogenatom or a substituent bulkier than methyl,

at least one of R^(a) to R^(d) represents a substituent bulkier thanmethyl, and

R¹ to R¹² are the same or different and each represent a hydrogen atomor a hydrocarbon group.

-   3. A reagent according to Item 2, wherein at least one of R^(a) to    R^(d) in formula (1) is one member selected from the group    consisting of 1) a C₁-C₈-hydrocarbon group, 2) an oxygen-containing    substituent, 3) a nitrogen-containing substituent, 4) a halogen    atom, and 5) a halogenated hydrocarbon group.-   4. A method for determining the absolute configuration of a chiral    compound comprising analyzing a sample solution containing a reagent    according to Item 1 and the chiral compound by circular dichroism    spectrophotometry to determine the absolute configuration of an    asymmetric carbon of the chiral compound based on a sign of the    Cotton effect, the chiral compound having the following    characteristics:

(i) being capable of coordinating to the metalloporphyrin dimer as anactive ingredient, and

(ii) having a group capable of coordinating to the metalloporphyrindimer directly bonded to the asymmetric carbon atom, or having onecarbon atom separating the group capable of coordinating to themetalloporphyrin dimer and the asymmetric carbon atom.

-   5. A method according to Item 4, wherein the chiral compound is    selected from one member of the group consisting of 1) a primary    monoamine, 2) a secondary monoamine, 3) a monoalcohol, and 4) an    aminoalcohol.-   6. A method according to Item 4, wherein the circular dichroism    spectrophotometric measurement is conducted at −10° C. to 30° C.

The present invention relates to a reagent for determining the absoluteconfiguration of chiral compounds comprising as an active ingredient ametalloporphyrin dimer which has alkaline-earth metal ion as the centralmetal and which has a carbon chain-crosslinked metalloporphyrin dimerstructure in which at least one of the two porphyrin rings has asubstituent bulkier than methyl at at least one of the second carbonatoms from the carbon atom bonded to the crosslinking carbon chain alongthe outer periphery of the porphyrin ring.

The metalloporphyrin dimer contained as an active ingredient is notlimited as long as it satisfies the above-described conditions, andincludes compounds represented by formula (1):

In formula (1), M²⁺ and M′²⁺ represent a central metal. M²⁺ and M′²⁺ maybe the same or different and each represent an alkaline-earth metal ionsuch as Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. Among these, Mg²⁺ and Ca⁺ arepreferable, and Mg²⁺ is particularly preferable.

The metalloporphyrin dimer used in the invention has a structure suchthat two metalloporphyrin units are cross-linked by a carbon chain. Forexample, in the compound represented by formula (1), the twometalloporphyrin units are cross-linked by —(CH₂)_(n)—. The number ofcarbon atoms in the crosslinking carbon chain is not limited and isgenerally about 2 or 3, and preferably 2. In formula (1), for example, nis 2 or 3, and preferably 2.

In formula (1), each R^(a) to R^(d) is a substituent bonded to secondcarbon atoms from the carbon atom bonded to the cross-linking carbonchain along the outer periphery of the porphyrin ring. In themetalloporphyrin dimer used in the invention, at least one of the secondcarbon atoms of the porphyrin rings from the carbon atom bonded to thecrosslinking carbon chain along the outer periphery of the porphyrinring is substituted with a substituent bulkier than methyl.

In formula (1), R^(a) to R^(d) is the same or different and eachrepresent a hydrogen atom or a substituent bulkier than methyl so longas at least one of R^(a) to R^(d) represents a substituent bulkier thanmethyl.

A substituent bulkier than methyl means a substituent whose volume is aslarge as or larger than a methyl group. Examples of such substituentsinclude 1) C₁₋₈ hydrocarbon groups, 2) oxygen-containing substituents,3) nitrogen-containing substituents, 4) halogen atoms, and 5)halogenated hydrocarbon groups, etc.

Examples of 1) C₁₋₈ hydrocarbon groups include linear or branched alkylgroup such as methyl, ethyl, propyl, n-butyl, isobutyl and the like. Thenumber of carbon atoms of the hydrocarbon group is preferably 1 to about4.

Examples of 2) oxygen-containing substituents include esters,carboxylalkyls, etc. Examples of esters include methyl esters, ethylesters, etc. Examples of carboxylalkyls include carboxymethyl, etc.

Examples of 3) nitrogen-containing substituents include aminos, amides,2-aminoethyl, etc.

Examples of 4) halogen atoms include Cl, Br, F, etc.

Examples of 5) halogenated hydrocarbon groups include chloromethyl,chloroethyl, chloropropyl, chlorobutyl, etc.

In formula (1), R¹ to R¹² are the same or different and each represent ahydrogen atom, a hydrocarbon group, etc. Examples of hydrocarbon groupsinclude a linear or branched alkyl groups such as methyl, ethyl, propyl,n-butyl, isobutyl, etc. The number of carbon atoms in the hydrocarbongroups represented by R¹ to R¹² is not limited and is generally 1 toabout 10, and preferably 1 to about 4.

A compound represented by formula (2) can be preferably used as themetalloporphyrin dimer used in the invention. Hereinafter, the compoundrepresented by formula (2) is sometimes referred to as “Compound 1.”

The reagent of the invention may contain additional ingredients withinranges in which the desired effect of the invention can be attained.

The method for producing the metalloporphyrin dimer to be used in thepresent invention is not limited, and the metalloporphyrin dimer can besynthesized according to known methods. For example, themetalloporphyrin dimer can be produced by a method for introducingalkaline-earth metal ions as central metals into unmetalated porphyrindimer) (which, hereinafter, is sometimes referred to as “a free baseporphyrin dimer”) by reacting the free base porphyrin dimer with analkaline-earth metal halide (for example, MgBr₂.Et₂O, etc.) in thepresence of basic compounds such as triethylamine, pyridine,methylpyridine, dimethylpyridine, diazine, methyldiazine, pyrazine,ethylpyrazine, pyrimidine, piperazine, morpholine, etc.

The free base porphyrin dimer can be produced by known methods. Forexample, the method proposed by V. V. Borovkov, J. M. Lintuluoto and Y.Inoue (Helv. Chem. Acta., 1999, 82, 919-934); and V. V. Borovkov, J. M.Lintuluoto and Y. Inoue (Synlett., 1998, 768), etc., can be mentioned.

An alkaline-earth metal ion can also be introduced to the free baseporphyrin dimer by a known method such as a method proposed by, forexample, J. S. Lindsey and J. N. Woodford (Inorg. Chem. 1995, 34,1063-1069).

The absolute configuration of an asymmetric carbon atom of a chiralcompound can be determined from the sign of the Cotton effect revealedby the analysis of circular dichroism spectrophotometry of the reagentof the invention and the chiral compound. The chiral compound has thefollowing characteristics (1) and (2).

(1) a chiral compound which can be coordinated to the metalloporphyrindimer as an active ingredient, and

(2) a chiral compound in which the group capable of coordinating to themetalloporphyrin dimer and asymmetric carbon are directly bonded to eachother or a chiral compound in which one carbon atom separates the groupthat can be coordinated to the metalloporphyrin dimer and the asymmetriccarbon.

The present invention includes a method for determining the absoluteconfiguration of the asymmetric carbon atom of a chiral compound basedon the sign of the Cotton effect revealed by the analysis of circulardichroism spectrophotometry of a sample solution containing theabove-mentioned reagent and the chiral compound. The chiral compound hasthe following characteristics (1) and (2).

(1) the chiral compound which can be coordinated to the metalloporphyrindimer as an active ingredient; and

(2) the chiral compound in which the group that can be coordinated tothe metalloporphyrin dimer and the asymmetric carbon atom are directlybonded to each other, or the chiral compound in which one carbon atomseparates the group that can be coordinated to the metalloporphyrindimer and the asymmetric carbon atom.

Examples of compounds that can be coordinated to the metalloporphyrindimer include compounds containing basic groups such as an amino group,a hydroxyl group, etc. as a group capable of coordinating to themetalloporphyrin dimer. More specifically, 1) primary monoamines, 2)secondary monoamines, 3) monoalcohols, 4) aminoalcohols, etc. In thecase of an aminoalcohol, the amino group readily coordinates to themetalloporphyrin dimer, thus determining the absolute configuration ofan asymmetric carbon atom directly bonded to the amino group or anasymmetric carbon atom separated by one carbon atom from the aminogroup.

The method of the present invention can determine the absoluteconfiguration of a chiral compound in which the group that can becoordinated to a central metal of the metalloporphyrin dimer as anactive ingredient and the asymmetric carbon atom are bonded to eachother, or a chiral compound in which one carbon atom separates the groupthat can be coordinated to a central metal of the metalloporphyrin dimerand the asymmetric carbon atom. For example, 2-butanol, 1-phenylethanol,1-phenylethylamine, etc. are equivalent to the chiral compounds in whichthe group that can be coordinated to a central metal of themetalloporphyrin dimer and the asymmetric carbon atom are directlybonded to each other. 2-methyl-1-butylamine, etc. can be mentioned as anexample of a chiral compound in which one carbon atom is present betweenthe group that can be coordinated to a central metal of themetalloporphyrin dimer and the asymmetric carbon atom.

In the case of a chiral compound having two or more asymmetric carbonatoms, such as borneol, menthol, etc., the absolute configuration of anasymmetric carbon atom directly bonded to the group that can becoordinated to a central metal of the porphyrin dimer, or an asymmetriccarbon atom that is separated by one carbon atom from the group that canbe coordinated to the porphyrin dimer can be determined.

The sample solution used for the circular dichroism spectrophotometricmeasurement contains the above-mentioned reagent and a chiral compound.Methods for preparing the sample solution are not limited, and thesolution can be prepared, for example, by a method of dissolving into asolvent the metalloporphyrin dimer as an active ingredient, the chiralcompound, etc.

A solvent which does not coordinate to the metalloporphyrin dimmer ispreferred for use in preparing the sample solution. Examples of suchsolvents include halogenated aliphatic hydrocarbons such as chloroform(CHCl₃), dichloromethane (CH₂Cl₂), dichloroethane (CH₂ClCH₂Cl),tetrachloroethane (CHCl₂CHCl₂), carbon tetrachloride (CCl₄), etc., andaliphatic hydrocarbons such as hexane, heptane, etc.

The concentrations of the chiral compound and metalloporphyrin dimer inthe sample solution are not limited. The lower concentration limit isnot limited as long as the first or second Cotton effect is detectable,and can be suitably determined depending on the type of metalloporphyrindimer, chiral compound, solvent, etc. to be used. The upperconcentration limit of the chiral compound and the metalloporphyrindimer in the sample solution are determined so that the ellipticityvalue of the first or second Cotton effect in the circular dichroismspectra (CD spectra) is, for example, about at least twice the noiselevel (for example, about 1 mdeg or more). Preferably, the ellipticityvalue of the first or second Cotton effect is the greatest possiblewithin the voltage range of a photomultiplier tube of up to −700 kV.More preferably, the ellipticity of the first or second Cotton effect isabout 10 mdeg to about 50 mdeg.

The concentration of the chiral compound in the sample solution can besuitably determined depending on type of chiral compound, etc. and isgenerally about 10⁻⁴ mol/l or more, preferably about 10⁻⁵ mol/l to about10⁻¹ mol/l, and more preferably about 10⁻⁴ mol/l to about 10⁻³ mol/l.

The concentration of the metalloporphyrin dimer in the sample solutioncan be suitably determined depending on type of chiral compound, etc.and is generally about 10⁻⁷ mol/l or more, and preferably about 10⁻⁶mol/l or more. The upper limit of the concentration of themetalloporphyrin dimer in the sample solution can be suitably determinedwithout limitation depending on type of chiral compound, etc. and isgenerally about 5×10⁻⁵ mol/l or less, and preferably about 5×10⁻6 mol/lor less.

The lowest concentrations of chiral compound in the sample solutionrequired for effectively detecting the Cotton effect are as follows.When Compound I represented by the above-mentioned formula (2) is usedas a reagent, an acyclic primary monoamine is about 10⁻⁶ mol/l, a cyclicaromatic monoamine is about 10⁻⁵ mol/l, a secondary monoamine is about10⁻⁶ mol/l, an aminoalcohol is about 10⁻⁴ mol/l and a monoalcohol isabout 10⁻¹ to about 10⁻² mol/l. Even when a monoalcohol is used, theCotton effect can be effectively detected by use of a metalloporphyrindimer of the above-mentioned concentration at about −10° C. to about 30°C.

The temperature for the circular dichroism spectrophotometricmeasurement is not limited as long as the Cotton effect is detectable.High-sensitivity measurements can be conducted at low temperatures, butthe Cotton effect can be detected even when the sample solution is notcooled.

The following cases 1 to 4 can be mentioned as examples showing morespecific analysis conditions.

<Case 1> Determination of the Absolute Configuration of PrimaryMonoamines:

Chloroform, dichloromethane, carbon tetrachloride, tetrachloroethane,hexane, heptane, dichloroethane, etc. are preferred as solvents. Theconcentration of an acyclic primary monoamine in the sample solution ispreferably 10⁻⁶ mol/l or more, and the concentration of a cyclicaromatic monoamine is preferably about 10⁻⁵ mol/l or more. When themagnesium porphyrin dimer represented by the above-mentioned formula (2)is used, its concentration in the sample solution is preferably about10⁻⁶ mol/l or more. Examples of primary monoamines as a measurementtarget include 2-butylamine, 1-phenylethylamine,1-(1-naphthyl)ethylamine, 1-cyclohexylethylamine, 2-methyl-1-butylamine,[endo-(1R)-1,7,7-trimethylbicyclo[2,2,1]heptane-2-amine], etc.

<Case 2> Determination of the Absolute Configuration of SecondaryMonoamines:

Chloroform, dichloromethane, carbon tetrachroide, tetrachloroethane,hexane, heptane, dichloroethane, etc. are preferable as solvents. Theconcentration of secondary monoamine in the sample solution ispreferably about 10⁻⁶ mol/l or more. When the magnesium porphyrin dimer(Compound 1) represented by the above-mentioned formula (2) is used, theconcentration thereof in the sample solution is preferably about 10⁻⁶mol/l or more. Examples of secondary monoamines as a measurement targetinclude N-methyl-1-phenylethylamine, etc.

<Case 3> Determination of the Absolute Configuration of Aminoalcohols:

Examples of preferable solvents include chloroform, dichloromethane,carbon tetrachloride, tetrachloroethane, hexane, heptane,dichlorioethane, etc. The concentration of aminoalcohol in the samplesolution is preferably about 10⁻⁴ mol/l or more. When Compound 1 isused, the concentration thereof in the sample solution is preferablyabout 10⁻⁶ mol/l or more. Examples of aminoalcohols as a measurementtarget include 1-amino-2-propanol, 2-amino-4-methyl-1-pentanol, etc.

<Case 4> Determination of the Absolute Configuration of Monoalcohols:

Methane dichloride or hexane is preferred as a solvent. Theconcentration of monoalcohol in the sample solution is preferably about10⁻² mol/l or more. When Compound 1 represented by the above-mentionedformula (2) is used, the concentration thereof in the sample solution ispreferably about 10⁻⁶ mol/l or more. Measurement temperature is usuallyset from −10° C. to 30° C., but the Cotton effect can be detected at thetemperature within the range when the sample solution is not cooled.Borneol, 2-butanol, 1-phenylethanol, etc. are examples of monoalcohol asmeasurement targets.

The circular dichroism spectra of sample solutions show two peaks (onemaximum value and one minimum value). Hereinafter, the peak occurring atlonger wavelength is sometimes referred to as the “first Cotton effect”,whereas the peak occurring at shorter wavelength is sometimes referredto as the “second Cotton effect”. The signs of the peaks may be positiveor negative. The sign of the first Cotton effect and the sign of thesecond Cotton effect are different from each other. When the CD spectraof (S)-(−)-1-phenylethanol shown in FIG. 5 are taken as an example, thesign of the first Cotton effect is positive (plus) and the sign of thesecond Cotton effect is negative (minus). The signs of the Cottoneffects of each optical isomer (for example, R isomer and S isomer) arereversed. For example, in the case of (S)-(+)-1-phenylethanol, the signsof the Cotton effect are reversed from the signs in the case shown inFIG. 1, and more specifically, the sign of the first Cotton effect isnegative and the sign of the second Cotton effect is positive. The signof either peak may be used for the determination of the absoluteconfiguration; the first Cotton effect is usually more easily detectedand thus is more preferably used.

In order to clarify a certain correspondence between the sign of eachCotton effect and the absolute configuration (R isomer or S isomer) ofthe asymmetric carbon atom of the chiral compound in the samplesolution, CD spectra of sample solutions each containing the reagent ofthis invention and one of various chiral compounds whose absoluteconfiguration is known were measured. The Table 1 shows the absoluteconfiguration and sign of various chiral compounds, and the sign of eachCotton effect in the sample solution. The metalloporphyrin containing Mgas a central metal (Compound 1) represented by the above-mentionedformula (2) is used as the metalloporphyrin dimer, and the concentrationin the sample solution was adjusted to 10⁻⁶ mol/l. The concentration ofeach chiral compound in the sample solution was adjusted to 10⁻⁴ mol/lwhen using amine and aminoalcohol, and was adjusted to 10⁻² mol/l whenusing alcohol.

TABLE 1 Absolute configuration Sign of second Sign of first and sign ofa Cotton effect Cotton effect Chiral Compound chiral compound (B

transition) (B_(∥) transition) 2-Butanol (R)-(−) + − 1-Phenylethanol(S)-(−) − + Borneol (1S, 2R)-(−) + − Menthol (1S, 2R, 5S)-(+) − + (1R,2S, 5R)-(−) + − 1-Naphthylethanol (R)-(+) + − Trans-2- (1S, 2R) − +phenylcyclohexanol Cis-verbenol (1S) − + Diacetone-D- (3S) − + glucose2-Butylamine (R)-(−) + − 1-Phenylethylamine (R)-(+) + − 2-Methyl-1-(S)-(−) − + butylamine 1-Amino-2-propanol (S) − +

As is evident from Table 1, there is a certain correspondence betweenthe sign of each Cotton effect and the steric configuration of theasymmetric carbon atom at the α or β position of the amino group orhydroxy group. More specifically, a positive sign of the first Cottoneffect corresponds to an (S) absolute configuration at the asymmetriccarbon at the above-mentioned α or β position. In contrast, a negativesign of the first Cotton effect corresponds to an (R) absoluteconfiguration at the asymmetric carbon at the above-mentioned α or βposition. From this correlation and the sign of the Cotton effect of asample solution whose absolute configuration is unknown can bedetermined, the absolute configuration of a chiral compound in suchsolution.

In Table 1, the B_(∥) transition occurs when the moments (transitionmoments) of two porphyrin rings are aligned in the direction of bondingthe porphyrin rings and the resulting absorption band is termed theB-band (or Soret band). The B_(⊥) transition is a transition occurringwhen the moments of two porphyrin rings are in directions perpendicularto the direction of bonding the porphyrin rings and the resultingabsorption band is also the B-band. In FIG. 2, the direction of themoments of an “achiral” metalloporphyrin dimer to which a chiralcompound is not coordinated is shown by solid double-headed arrows.

In contrast, in either the B_(∥) transition or the B_(⊥) transition, thedirections of the moments of the two porphyrin rings of a chiralmetalloporphyrin dimer to which a chiral compound is coordinated areslightly misaligned with respect to each other, as compared to theachiral porphyrin dimer, as shown by the dotted double-headed arrows inFIG. 2.

Hereinafter, the mechanism of the induced Cotton effect in the presentinvention is described. A chiral compound is coordinated to a centralmetal of the metalloporphyrin dimer in a solution containing themetalloporphyrin dimer as an active ingredient and the above-mentionedchiral compound. The chiral compound may be coordinated to either one ofthe two central metals. For example, when either one of the twoporphyrin rings has at least one substituent bulkier than methyl, thechiral compound is coordinated to the central metal of the otherporphyrin ring instead of the porphyrin ring having such a substituent.More specifically, the chiral compound is coordinated to M′²⁺ when onlyR^(a) of the compound represented by the above-mentioned formula (1) isa substituent bulkier than methyl. The metalloporphyrin dimer shows aconformational change from syn to anti with such orientation of a chiralcompound, and at the same time, asymmetry is induced in the anticonformer, whereby circular dichroism is exhibited as shown in FIG. 1.The syn conformer of Compound 1 represented by the above-mentionedformula (2) is shown below.

The mechanism of the asymmetry induction is illustrated in FIG. 3 whichshows the porphyrin rings are twisted. FIG. 3 illustrates the case wherean S-amine is used as the chiral compound. Thus it can be understoodthat the porphyrins' orientation is twisted by the steric hindrancebetween the bulkiest substituent (X) bonded to the α or β carbon atom (αcarbon atom in FIG. 3) of the chiral compound as a ligand and thesubstituent bulkier than a methyl group (Et: ethyl in FIG. 3) of theporphyrin ring to which the chiral compound is not coordinated, therebyproducing circular dichroism based on the exciton interaction betweenthe porphyrins.

According to the CD exciton-chirality method (Harada, N., Nakanishi, K.,“Circular Dichroic Spectroscopy-Exciton Coupling in OrganicStereochemistry, University Science Books, Mill Valley, 1983; Nakanishi,K., Berova, N., “In Circular Dichroism; Principles and Applications”,Woody, R., Ed, VCH Publishers; New York, 1994; pp. 361-398), a clockwiseorientation when viewed from the front of two interacting electronictransition moments produces positive chirality, while a counterclockwiseorientation leads to negative chirality.

For example, when a chiral compound of (S) absolute configuration iscoordinated to the metalloporphyrin dimer, the porphyrin is clockwisetwisted when viewed from the front (see FIG. 3), and thus the sign ofthe first Cotton effect is “positive”. In contrast, when a chiralcompound of (R) absolute configuration is coordinated to themetalloporphyrin dimer, the porphyrin is counterclockwise twisted whenviewed from the front, and thus the sign of the first Cotton effect is“negative”.

The method of the invention is used for circular dichroism (CD)spectrophotometric measurement of a sample solution containing a chiralcompound, a metalloporphyrin, etc. More specifically, according to themethod of the invention, the absolute configuration of chiral compoundscan be determined by the sign of the Cotton effect of the circulardichroism spectra obtained when a chiral compound as a measurementtarget is coordinated to the metalloporphyrin dimer. According to thismethod, the absolute configuration of chiral compounds can be directlyobserved without the introduction of any specifically modified group.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the result obtained in Example 1 and shows CD spectra of asample solution containing borneol.

FIG. 2 schematically illustrates the dipole moment directions of the Bband as the maximum absorption band of a porphyrin dimer, in which ametalloporphyrin dimer is schematically illustrated with an ellipsedepicting each porphyrin ring and a straight or kinked line depictingthe crosslinking carbon chain. The two ellipses (porphyrin rings) do notexist in the same plane but are located in front and rear. In order toclarify the relative positions of the two ellipses, the front ellipse isshown thickened. The chiral structure has a clockwise chirality.

FIG. 3 schematically illustrates a mechanism of asymmetry induction in amagnesium porphyrin dimer, in which the front porphyrin ring of the twoporphyrin rings is shown thickened.

FIG. 4 shows UV-visible absorption spectra of Compound 1 prepared inProduction Example 1 (shown by the thick line “a”). (R)-2-butylamine wasadded to a solution of Compound 1, which changes the spectra (shown bythe thin line “b”). This shows that there is a certain interactionbetween Compound 1 and (R)-2-butylamine.

FIG. 5 shows CD spectra of a sample solution containing(S)-(−)-1-phenylethanol measured in Example 2 (shown by the solid line)and a sample solution containing (S)-(−)-1-phenylethanol measured inComparative Example 1 (shown by the dotted line).

FIG. 6 shows CD spectra (shown by the solid line) of a sample solutioncontaining (S)-(−)-1-phenylethylamine measured in Example 2 and shows CDspectra (shown by the dotted line) of a sample solution containing(S)-(−)-1-phenylethylamine measured in Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail withreference to Examples, but is not limited thereto.

A JASCO J-720WI spectropolarimeter was used to measure CD spectra.

PRODUCTION EXAMPLE 1

A magnesium porphyrin dimer was produced according to the followingscheme.

A free base porphyrin dimer (Compound 2, 1.9 mg, 1.73 μmol) wasdissolved in anhydrous CH₂Cl₂ (1 ml); triethylamine (9.5 μl, 68 μmol)was further added thereto, and (CH₂H₅)₂O.MgBr₂ (10 mg, 35 μmol) wasfurther added. The obtained mixture was stirred at room temperature forone hour. 10 ml of CH₂Cl₂ was added to this mixture to dilute it and thereaction mixture was washed with 5 ml of water, dried with anhydroussodium sulfate and filtered. The obtained filtrate was concentrated to 1ml. A column charged with basic activated alumina (manufacture by Merck,active aluminium oxide 90) was used for column chromatographicpurification. CH₂Cl₂ was used as a first eluate (unreacted free-base wasnot detected), and a mixed solvent with 1:1 ratio of CH₂Cl₂ to acetonewas used as a second eluate. 1.95 mg of Compound 1 (98% yield withrespect to free base porphyrin dimer Compound 2) was obtained by dryingthe second eluate by evaporation. UV-visible absorption spectra inCH₂Cl₂ of the obtained compound are shown in FIG. 4 (shown by the thickline “a”). Absorption maxima: λ_(max) was as follows: λ_(max)/nm: 601(3.83), 557 (4.18), 416 (5.22), 404 (5.29). The values in brackets showthe logarithm of the molar absorption coefficient for each maximumabsorption: log_(ε).

FIG. 4 also shows UV-visible absorption spectra of the solution obtainedby further adding (R)-2-butylamine to the CH₂Cl₂ solution of Compound 1(shown by the thin line “b”).

Example 1

A CH₂Cl₂ solution containing about 10⁻⁶ mol/l of Compound 1 and about10⁻¹ mol/l of (1S, 2R)-(−)-borneol was prepared in a 3 ml cell (opticalpath length: 1 cm), and circular dichroism spectra were observed overthe region from 350 to 500 nm at 22 to 23° C. FIG. 1 shows the result.

As is evident from FIG. 1, from the negative sign of the First Cottoneffect at longer wavelengths, the (R) absolute configuration of theasymmetric carbon atom bonded to the hydroxyl group of borneol wasconfirmed.

Example 2

Circular dichroism spectra were measured in the same manner as inExample 1 except that 10⁻² mol/l of (S)-(−)-1-phenylethanol was usedinstead of borneol. FIG. 5 (solid line) shows the results.

In this case, from the positive sign of the first Cotton effect, the (S)absolute configuration was confirmed.

Example 3

Circular dichroism spectra were measured in the same manner as inExample 1 except that 10⁻⁴ mol/l of phenylethylamine was used instead ofborneol. FIG. 6 (solid line) shows the result. In this case, from thenegative sign of the first Cotton effect, the (R) absolute configurationwas confirmed.

Comparative Example 1

Circular dichroism spectra of a sample solution containing1-phenylethanol were measured in the same manner as in Example 2 exceptthat a zinc porphyrin dimer (approximately 10⁻⁶ mol/l) represented byformula below was used instead of Compound 1 as a reagent to determinethe absolute configuration of a chiral compound.

FIG. 5 (dotted line) shows the results. The absolute configuration of1-phenylethanol was not determined since the Cotton effect was notdetected. However, the Cotton effect was detected when the samplesolution was cooled to about −80° C.

{μ-{{5,5′-(ethane-1,2-diyl)bis[2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinato](4-)}-_(κ)N²¹,_(κ)N²², _(κ)N²³, _(κ)N²⁴: N^(21′), _(κ)N^(22′), _(κ)N^(23′),_(κ)N^(24′)}}-dizinc Comparative Example 2

Circular dichroism spectra of a sample solution containing1-phenylethylamine were measured in the same manner as in Example 3except that the zinc porphyrin dimer (approximately 10⁻⁶ mol/l) used inComparative Example 1 was used instead of Compound 1 as a reagent fordetermining the absolute configuration of a chiral compound.

The result is shown by the dotted line in FIG. 6. The absoluteconfiguration of 1-phenylethylamine was not determined since the Cottoneffect was not detected. However, the Cotton effect was detected byadjusting the concentration of 1-phenylethylamine to 1×10⁻³ mol/l.

INDUSTRIAL APPLICABILITY

With respect to chiral compounds which can be coordinated to ametalloporphyrin dimer, the present invention allows directdetermination of the absolute configuration of an asymmetric carbon atomwhich is directly bonded to a group capable of coordinating to themetalloporphyrin dimer, or an asymmetric carbon atom separated by onecarbon atom from the coordinating group. In the case of a compoundhaving two or more asymmetric carbons, such as menthol, the absoluteconfiguration of the asymmetric carbon to which a group coordinating toa metalloporphyrin dimer is bonded or an asymmetric carbon having onecarbon atom separating it from the coordinating group can be determined.

According to the present invention, the absolute configuration of achiral compound can be determined without complicated processes such ascooling the sample solution.

The present invention provides an extremely sensitive method, and thustrace amounts of chiral compounds are enough for determination ofabsolute configuration. The amount of metalloporphyrin dimer required isalso very small. When an acyclic primary monoamine is taken as anexample, the method disclosed in Japanese Unexamined Patent PublicationNo. 2001-220392 requires at least 10⁻³ mol/l of amine for thedetermination of the absolute configuration, while according to themethod of the invention, at least 10⁻⁶ mol/l of amine is enough.

The invention eliminates the necessity of introducing any specificmodified groups into the chiral compound for measurement. Therefore,chiral compounds can be easily recovered as required. For example, acomplex composed of the metalloporphyrin dimer and a chiral compounddecomposes into the free base porphyrin dimer and the hydrochloride ofthe chiral amine by stirring the sample solution after measurement andabout 2 mol/l of aqueous hydrochloric acid solution, thus allowingrecovery of the chiral compound by separation. The free base porphyrindimer recovered by separation can be recycled by introducing analkaline-earth metal ion again thereinto.

According to the present invention, the absolute configuration of achiral compound can be determined quite rapidly. The period required forpreparing a sample and measuring its CD spectra is less than 10 minutesunder a certain condition.

The detection of Cotton effects, i.e., the measurement of CD spectra, isusually carried out in the region from 400 to 450 nm. Most chiralcompounds have absorptions up to 400 nm, and thus the peak which showsthe Cotton effect and the peaks shown by the chiral compounds themselvesdo not overlap. Therefore, the invention allows the determination of theabsolute configuration of a very wide variety of chiral compounds.

Furthermore, the absolute configuration of non-crystalline compounds canbe determined by the present invention.

1. A reagent for determining the absolute configuration of a chiralcompound, the reagent containing as an active ingredient ametalloporphyrin dimer comprising two porphyrin rings, wherein themetalloporphyrin dimer has Mg²⁺ as a central metal and has a carbonchain-crosslinked structure in which at least one of the two porphyrinrings has a substituent bulkier than methyl at at least one carbon atomlocated two carbon atoms away from a carbon atom bonded to thecrosslinking carbon chain along the outer periphery of the porphyrinring, and wherein an induced Cotton effect can be detected by saidmetalloporphyrin dimer in the case of a chiral compound of a monoalcoholwithout cooling to about −80° C.
 2. A reagent for determining theabsolute configuration of a chiral compound according to claim 1,wherein the carbon chain-crosslinked metalloporphyrin dimer is ametalloporphyrin represented by formula (1):

wherein M²⁺ and M′²⁺ are Mg²⁺, n is 2 or 3, R^(a) to R^(d) are the sameor different and each represent a hydrogen atom or a substituent bulkierthan methyl, at least one of R^(a) to R^(d) represents a substituentbulkier than methyl, and R¹ to R¹² are the same or different and eachrepresent a hydrogen atom or a hydrocarbon group.
 3. A reagent accordingto claim 2, wherein at least one of R^(a) to R^(d) in formula (1) is onemember selected from the group consisting of 1) a C₁-C₈ hydrocarbongroup, 2) an oxygen-containing substituent, 3) a nitrogen-containingsubstituent, 4) a halogen atom, and 5) a halogenated hydrocarbon group.4. A method for determining the absolute configuration of a chiralcompound comprising analyzing a sample solution containing a reagentaccording to claim 1 and the chiral compound by circular dichroismspectrophotometry to determine the absolute configuration of anasymmetric carbon of the chiral compound based on a sign of the Cottoneffect, the chiral compound having the following characteristics: (i)being capable of coordinating to the metalloporphyrin dimer as an activeingredient, and (ii) having a group capable of coordinating to themetalloporphyrin dimer directly bonded to the asymmetric carbon atom, orhaving one carbon atom separating the group capable of coordinating tothe metalloporphyrin dimer and the asymmetric carbon atom, wherein aninduced Cotton effect can be detected by said metalloporphyrin dimer inthe case of a chiral compound of a monoalcohol without cooling to about−80° C.
 5. A method according to claim 4, wherein the chiral compound isselected from one member of the group consisting of 1) a primarymonoamine, 2) a secondary monoamine, 3) a monoalcohol, and 4) anaminoalcohol.
 6. A method according to claim 4, wherein the circulardichroism spectrophotometric measurement is conducted at −10° C. to 30°C.
 7. A method for determining an absolute configuration of a chiralcompound, comprising: providing a chiral compound (i) being capable ofcoordinating to the metalloporphyrin dimer according to claim 1 as anactive ingredient and (ii) having a group capable of coordinating to themetalloporphyrin dimer directly bonded to an asymmetric carbon atom, orhaving one carbon atom separating the group capable of coordinating tothe metalloporphyrin dimer and the asymmetric carbon atom; providing asample solution containing the chiral compound and a reagent, saidreagent containing as an active ingredient a metalloporphyrin dimer,wherein the metalloporphyrin dimer has two porphyrin rings and acrosslinking carbon chain which bonds the two porphyrin rings, eachporphyrin ring has Mg²⁺ as a central metal, and at least one of the twoporphyrin rings has a substituent bulkier than methyl at at least one ofcarbon atoms which are positioned secondarily along the outer peripheryof the porphyrin ring based on a carbon atom bonded to the crosslinkingcarbon chain; and subjecting the sample solution to circular dichroismspectrophotometry to determine the absolute configuration of anasymmetric carbon of the chiral compound based on a sign of the Cottoneffect, wherein an induced Cotton effect can be detected by saidmetalloporphyrin dimer in the case of a chiral compound of a monoalcoholwithout cooling to about −80° C.
 8. The method according to claim 7,wherein the chiral compound is selected from one member of the groupconsisting of 1) a primary monoamine, 2) a secondary monoamine, 3) amonoalcohol, and 4) an aminoalcohol.
 9. The method according to claim 7,wherein the carbon chain-crosslinked metalloporphyrin dimer is ametalloporphyrin represented by formula (1):

wherein M²⁺ and M′²⁺ are Mg²⁺; n is 2 or 3; R^(a) to R^(d) are the sameor different and each represent a hydrogen atom or a substituent bulkierthan methyl; at least one of R^(a) to R^(d) represents a substituentbulkier than methyl; and R¹ to R¹² are the same or different and eachrepresent a hydrogen atom or a hydrocarbon group.
 10. The methodaccording to claim 7, wherein the circular dichroism spectrophotometricmeasurement is conducted at −10° C. to 30° C.
 11. A reagent fordetermining an absolute configuration of a chiral compound, the reagentcontaining as an active ingredient a metalloporphyrin dimer, wherein themetalloporphyrin dimer has two porphyrin rings and a crosslinking carbonchain which bonds the two porphyrin rings, each porphyrin ring has Mg²⁺as a central metal, and at least one of the two porphyrin rings has asubstituent bulkier than methyl at at least one of carbon atoms whichare positioned secondarily along the outer periphery of the porphyrinring based on a carbon atom bonded to the crosslinking carbon chain,wherein an induced Cotton effect can be detected by saidmetalloporphyrin dimer in the case of a chiral compound of a monoalcoholwithout cooling to about −80° C.
 12. A mixture of a chiral compound anda reagent for determining an absolute configuration of the chiralcompound, said reagent containing as an active ingredient ametalloporphyrin dimer having two porphyrin rings and a crosslinkingcarbon chain which bonds the two porphyrin rings, each porphyrin ringhas Mg²⁺ as a central metal, and at least one of the two porphyrin ringshas a substituent bulkier than methyl at at least one of carbon atomswhich are positioned secondarily along the outer periphery of theporphyrin ring based on a carbon atom bonded to the crosslinking carbonchain, wherein an induced Cotton effect can be detected by saidmetalloporphyrin dimer in the case of a chiral compound of a monoalcoholwithout cooling to about −80° C.
 13. The mixture according to claim 12,wherein the carbon chain-crosslinked metalloporphyrin dimer is ametalloporphyrin represented by formula (1):

wherein M²⁺ and M′²⁺ are Mg²⁺; n is 2 or 3; R^(a) to R^(d) are the sameor different and each represent a hydrogen atom or a substituent bulkierthan methyl; at least one of R^(a) to R^(d) represents a substituentbulkier than methyl; and R¹ to R¹² are the same or different and eachrepresent a hydrogen atom or a hydrocarbon group.
 14. The mixtureaccording to claim 12, wherein the chiral compound (i) is capable ofcoordinating to the metalloporphyrin dimer as an active ingredient and(ii) has a group capable of coordinating to the metalloporphyrin dimerdirectly bonded to an asymmetric carbon atom, or has one carbon atomseparating the group capable of coordinating to the metalloporphyrindimer and the asymmetric carbon atom.
 15. The mixture according to claim14, wherein the chiral compound is selected from one member of the groupconsisting of 1) a primary monoamine, 2) a secondary monoamine, 3) amonoalcohol, and 4) an aminoalcohol.
 16. The mixture according to claim12, which is a solution.